An important aim in elevator systems is to maximize passenger safety. Free fall of the elevator car must be prevented and the motion must not reach an uncontrolled acceleration and consequent uncontrolled deceleration of motion. Sudden stops occurring even with a relatively low kinetic energy may cause injuries to passengers.
FIG. 1 illustrates a prior-art roped elevator system and associated common prior-art safety devices. The basic parts constituting the elevator system are an elevator shaft 100, an elevator car 102 traveling in it, ropes 116–120 connected to the elevator car, a traction sheave 106 and a counterweight 104. The counterweight is so designed that its weight corresponds to the weight of the elevator car 102 and the weight of the associated mechanical equipment on the car side plus half the weight of the nominal load. With this arrangement, the weight difference between the car and counterweight sides is half the nominal load, unless the elevator car is overloaded. Nominal load refers to the load carried in the elevator car. Extending on the side or back walls of the elevator shaft are at least two guide rails 122, 124, the function of which is to keep the elevator car in position in the frontways and backways directions in relation to the counterweight. In the figure, the elevator car is provided with safety gears 154–156, which have brake shoes that safety gears 154–156, which have brake shoes that brake the car by gripping the guide rails 122 and 124. The traction sheave 106 is connected by an axle 107 to a power transmission means 109, which may also contain a gear system. In this case, the elevator machine is a geared machine. The elevator machine may also be gearless. Connected by an axle 108 to the power transmission means 109 is a motor 110. The motor 110 is controlled by a control system 114 over a control cable 112. The motor may be a single-speed, dual-speed or variable-speed motor. The control system 114 may control the motor torque in a stepless manner e.g. by a control arrangement based on variable voltage and variable frequency (V3F). Also connected to the control system 114 are systems for handling car calls and button control. The axle 108 is provided with brakes 160–162, which have brake drums for braking the axle 108. The brakes 160–162 are connected to the control system 114 via a control cable 111. The traction sheave is equipped with a motion measuring device 115, which is e.g. a tachometer. The motion measuring device 115 is connected to the control unit by a cable 116.
The authorities in different countries have different regulations concerning the safety of elevators. The basic principle is that the brake system of the elevator should be able to stop the elevator car from the nominal speed and keep it immovable at a floor even in overload situations, where the elevator car has an excessive load. The brake system should have a fault tolerance such that one mechanical fault will not make the brake system completely inoperative.
For example, according to European safety regulation EN81-1:1998 the brake must be mechanically doubled so that when one part fails, the other half of the brake is still operative. A mechanically doubled brake must be able to stop a 125-% load moving downwards at nominal speed. Nominal speed refers to the maximum speed of motion that can be reached by the motor of the machine.
The condition and operation of the brakes of an elevator are generally only tested in connection with maintenance visits. In the case of single- and dual-speed elevators, the condition of the brakes can be detected from an impaired stopping accuracy. In the case of variable-speed elevators employing electronic adjustment, operativeness of the brakes can not be so easily detected because the brakes do not necessarily have a significant effect on stopping accuracy. For this reason, the operativeness of the brakes of variable-speed elevators is completely dependent on the maintenance program. It is additionally possible that the elevator serviceman adjusts the brakes incorrectly to reduce the disturbing noise produced by the brakes. However, the operation of the brakes is of primary importance for reasons of safety. Due to the aforesaid circumstances, maintenance of the brakes of elevators requires considerable carefulness and it has a significant effect on safety.